With the recent rapid development of portable and cordless electronic devices such as audio-visual (AV) devices and personal computers, there is an increasing demand for secondary batteries having a small size, a light weight and a high energy density as a power source for driving these electronic devices. Under these circumstances, lithium ion secondary batteries having advantages such as a high charge/discharge voltage and a large charge/discharge capacity have been noticed.
Hitherto, as positive electrode (cathode) active substances useful for high energy-type lithium ion secondary batteries exhibiting a 4 V-grade voltage, there are generally known LiMn2O4 having a spinel structure, and LiCoO2, LiCO1-xNixO2 and LiNiO2 having a layered rock-salt type structure, or the like. Among these active substances, LiCoO2 is more excellent because of a high voltage and a high capacity thereof, but has problems such as a high production cost due to a less amount of a cobalt raw material supplied, and a poor environmental safety upon disposal of batteries obtained using the substance. In consequence, there have now been made earnest studies on lithium manganate having a spinel type structure (basic composition: LiMn2O4; this is hereinafter defined in the same way) which is produced by using, as a raw material, manganese having a large supply amount, a low cost and a good environmental compatibility. Further, although the layered rock-salt type structure has a two-dimensional diffusion path, the spinel structure has a three-dimensional Li diffusion path. Therefore, it is expected that the latter spinel structure is used as a positive electrode active substance in the applications requiring a large electric current, in particular, in the applications of a large secondary batteries for automobiles.
As is known in the art, the lithium manganate particles may be obtained by mixing a manganese compound and a lithium compound at a predetermined ratio and then calcining the resulting mixture in a temperature range of 700 to 1000° C.
However, when the lithium manganate is highly enhanced in crystallizability in order to obtain a crystal structure suitable for an enhanced performance of the battery, the resulting lithium manganate particles have an octahedral shape with a low packing rate as an automorphic shape of the cubic spinel structure as shown in FIG. 7. Therefore, when using the lithium manganate particles having such an octahedral structure as a positive electrode active substance for lithium ion secondary batteries, there tends to arise such a problem that the obtained battery is deteriorated in capacity. In addition, the battery tends to be deteriorated in charge/discharge cycle characteristics and storage characteristics under high-temperature conditions. The reason therefor is considered to be that when charge/discharge cycles are repeated, the crystal lattice is expanded and contracted owing to desorption and insertion behavior of lithium ions in the crystal structure to cause change in volume of the crystal, which results in occurrence of breakage of the crystal lattice, deteriorated current collecting property of the electrode or elution of manganese in an electrolyte solution.
At present, in the lithium ion secondary batteries using the lithium manganate particles, it has been strongly required that the positive electrode active substance is packed in an electrode with a high packing density, the electrode formed from the positive electrode active substance has a low electric resistance, and the resulting batteries are free from deterioration in charge/discharge capacity due to repeated charge/discharge cycles and improved in their characteristics, in particular, under high-temperature conditions.
In order to improve the charge/discharge cycle characteristics of the batteries under high-temperature conditions, it is necessary that the positive electrode active substance used therein which comprises the lithium manganate particles has an excellent packing property and an appropriate particle size, and further is free from elution of manganese therefrom. To meet these requirements, there have been proposed the method of suitably controlling a particle size and a particle size distribution of the lithium manganate particles; the method of obtaining the lithium manganate particles having a high crystallinity by controlling a calcination temperature thereof (Patent Document 1); the method of adding different kinds of elements to the lithium manganate particles to strengthen a bonding force between crystals thereof (Patent Documents 2 to 4); the method of subjecting the lithium manganate particles to surface treatment or adding additives thereto to suppress elution of manganese therefrom (Patent Documents 5 and 6); or the like.
Also, in Patent Document 7, there is described the method of reducing an electric resistance of a positive electrode active substance by improving a crystallizability of the lithium manganate particles and thereby obtaining particles having an octahedral shape or a generally octahedral shape.
Patent Document 1: Japanese Patent Application Laid-Open (KOAKI) No. 2001-206722
Patent Document 2: Japanese Patent Application Laid-Open (KOAKI) No. 2000-215892
Patent Document 3: Japanese Patent Application Laid-Open (KOAKI) No. 2002-145617
Patent Document 4: Japanese Patent Application Laid-Open (KOAKI) No. 2008-251390
Patent Document 5: Japanese Patent Application Laid-Open (KOAKI) No. 2000-58055
Patent Document 6: Japanese Patent Application Laid-Open (KOAKI) No. 2002-308628
Patent Document 7: Japanese Patent Application Laid-Open (KOAKI) No. 2000-113889